1. Field of the Invention
This invention relates to an image-forming method and an image-forming apparatus which are applicable to laser beam color printers and color copying apparatus, and more particularly to a high-speed full-color image-forming method and image-forming apparatus therefor.
2. Related Background Art
As photosensitive members used conventionally in image-forming apparatus of this type are roughly grouped into organic members and inorganic members, as exemplified by OPC photosensitive members and a-Si photosensitive members. These techniques are described below.
Organic Photoconductor (OPC) Photosensitive Member
In recent years, as photoconductive materials for electrophotographic photosensitive members development on various organic photoconductive materials has advanced, and especially function-separated photosensitive members having a charge generation layer and a charge transport layer which are superposed have already been put into practical use and are set in copying machines and laser beam printers.
In these photosensitive members, however, it has been considered to be a problem that they commonly have a low durability. The durability is roughly grouped into durability on electrophotographic physical properties (i.e., running performance) concerning sensitivity, residual potential, charging performance and faint images (blurred image), and mechanical durability concerning wear and scratches of photosensitive member surfaces which are caused by rubbing.
Of these, with regard to the durability on electrophotographic physical properties (running performance), in particular, faint images, are known to be caused by deterioration of charge-transporting materials contained in photosensitive member surface layers which are due to active substances such as ozone and NOx generated from corona charging assemblies.
With regard to the mechanical durability, it is known to be caused by paper, cleaning members (such as a blade or a roller) and toners which come into contact with, and rub against, photosensitive layers.
In order to improve the the durability on electrophotographic physical properties (running performance), it is important to use charge-transporting materials that may be deteriorated with difficulty by the active substances such as ozone and NOx. It is known to select charge-transporting materials having a high oxidation potential.
In order to improve the mechanical durability, it is important to make the surface have a high lubricity and a low friction in order to withstand the rubbing with paper and cleaning members, and also to make the surface have good releasability in order to prevent toners from causing filming melt adhesion. It is known to incorporate surface layers with lubricating materials such as fluorine resin powder, fluorinated graphite and polyolefin resin powder.
If, however, the surface may extremely less wear, any moisture-absorptive substances produced by the active substances such as ozone and NOx may accumulate on the photosensitive member surface. As the result, the surface resistance lowers to make surface electric charges move in lateral directions to cause faint images (smeared images) in some cases.
Inorganic Photoconductor: Amorphous Silicon (a-Si) Photosensitive Member
In electrophotography, photoconductive materials that form photosensitive layers in photosensitive members are required to be highly sensitive, have a high SN ratio [light current (Ip)/dark current (Id)], have absorption spectra suited to spectral characteristics of irradiation light or electromagnetic waves, have a high response to light, have the desired dark resistance value and are harmless to human bodies when used. In particular, in the case of photosensitive members for image-forming apparatus, set in image-forming apparatus used in offices as business machines, the harmlessness in their use is an important point.
Photoconductive materials having good properties in these respects include amorphous silicon hydrides (hereinafter xe2x80x9ca-Si:Hxe2x80x9d). For example, Japanese Patent Publication No. 60-35059 discloses its application in photosensitive members for image-forming apparatus.
Now, the layer construction of photosensitive members is described with reference to FIGS. 3A to 3D, which diagrammatically illustrate the layer construction of photosensitive members used in image-forming apparatus. The following description is a general description of photosensitive members. Hence, it is a description on the background art and at the same time it is applicable also to photoconductive members usable in the image-forming apparatus of the present invention. Also, the layer construction of photosensitive members shown in FIGS. 3A to 3D shows a first example to a fourth example of the layer construction of photosensitive members used in the present invention and in conventional-image-forming apparatus.
A photosensitive member 1100 for image-forming apparatus as shown in FIG. 3A has a photosensitive member support 1101 and a photosensitive layer 1102 provided thereon. The photosensitive layer 1102 is formed of a-Si:H,X and is constituted of a photoconductive layer 1103 having photoconductivity.
A photosensitive member 1100 for image-forming apparatus as shown in FIG. 3B has a photosensitive member support 1101 and a photosensitive layer 1102 provided thereon. The photosensitive layer 1102 is formed of a-Si:H,X and is constituted of a photoconductive layer 1103 having photoconductivity and an amorphous silicon surface layer 1104.
A photosensitive member 1100 for image-forming apparatus as shown in FIG. 3C has a photosensitive member support 1101 and a photosensitive layer 1102 provided thereon. The photosensitive layer 1102 is formed of a-Si:H,X and is constituted of a photoconductive layer 1103 having photoconductivity, an amorphous silicon surface layer 1104 and a charge injection block layer 1105 of an amorphous silicon type.
A photosensitive member 1100 for image-forming apparatus as shown in FIG. 3D has a photosensitive member support 1101 and a photosensitive layer 1102 provided thereon. The photosensitive layer 1102 is constituted of a photoconductive layer 1103 consisting of a charge transport layer 1106 and a charge generation layer 1107 which are formed of a-Si:H,X, and an amorphous silicon surface layer 1104.
In FIGS. 3A to 3D, reference numerals 1106 each denote a free surface.
The image-forming apparatus photosensitive members making use of a-Si:H are commonly produced by heating conductive supports to 50xc2x0 C. to 400xc2x0 C. and forming photosensitive layers comprised of a-Si, on the supports by a film-forming process such as vacuum deposition, sputtering, ion plating, thermal CVD (chemical vapor deposition), photo-assisted CVD, plasma-assisted CVD (hereinafter xe2x80x9cPCVDxe2x80x9d). In particular, PCVD (i.e., a process in which material gases are decomposed by direct-current, high-frequency or microwave glow discharge to form a-Si deposited films on supports) is put into practical use as a preferable process.
Japanese Patent Application Laid-Open No. 56-83746 discloses an image-forming apparatus photosensitive member comprising a conductive support and an a-Si photoconductive layer containing halogen atoms as a constituent (hereinafter xe2x80x9ca-Si:Xxe2x80x9d). This publication reports that the incorporation of 1 to 40 atom % of halogen atoms into a-Si enables achievement of a high heat resistance and also electrical and optical properties preferable for a photoconductive layer of an image-forming apparatus photosensitive member.
Japanese Patent Application Laid-open No. 57-115556 also discloses a technique in which, in order to achieve improvements in electrical, optical and photoconductive properties such as dark resistivity, photoconductivity and response to light, service environmental properties such as moisture resistance, and stability with lapse of time, of a photoconductive member having a photoconductive layer constituted of an a-Si deposited film, a surface layer constituted of a non-photoconductive amorphous material containing silicon atoms and carbon atoms is provided on a photoconductive layer constituted of an amorphous material composed chiefly of silicon atoms.
Japanese Patent Application Laid-Open No. 60-67951 also discloses a technique concerning a photosensitive member provided with a light-transmitting insulating overcoat layer containing amorphous silicon, carbon, oxygen and fluorine. Japanese Patent Application Laid-open No. 62-168161 discloses a technique in which an amorphous material containing silicon atoms, carbon atoms and from 41 to 70 atom % of hydrogen atoms as constituents is used for a surface layer.
Japanese Patent Application Laid-Open No. 57-158650 still further discloses that an image-forming apparatus photosensitive member having a high sensitivity and a high resistance can be obtained by using in a photoconductive layer a-Si:H containing from 10 to 40 atom % of hydrogen and an absorption coefficient ratio of from 0.2 to 1.7 in respect of absorption peaks at 2,100 cmxe2x88x921 and 2,000 cmxe2x88x921 of an infrared absorption spectrum.
Japanese Patent Application Laid-Open No 60-95551 discloses a technique in which, aiming at an improvement in quality of images on an amorphous silicon photosensitive member, image-forming steps of charging, exposure, development and transfer are carried out maintaining the temperature at from 30 to 40xc2x0 C. in the vicinity of the photosensitive member surface, so as to prevent surface resistance from decreasing with absorption of moisture at the photosensitive member surface and prevent smeared images (high-humidity smearing) from occurring concurrently therewith.
These techniques have achieved improvements in electrical, optical and photoconductive properties and service environmental properties of image-forming apparatus photosensitive members and also have concurrently brought about an improvement in image quality.
Support
As supports used in image-forming apparatus photosensitive members, they may be conductive or may be electrically insulative. Conductive supports may include supports made of metals such as Al and Fe and alloys of these (e.g., stainless steel). Also usable are supports obtained by subjecting the surfaces of electrically Insulative supports such as films or sheets of synthetic resins and glass or ceramic sheets to photoconductive treatment at least on the side where the photosensitive layer is formed.
In FIGS. 3A to 3D for example, the supports 1101 used may have the shape of cylinders or sheetlike endless belts with a smooth surface or uneven surface.
Especially when images are recorded using coherent light such as laser light, in order to more effectively cancel any faulty images due to interference fringes appearing in visible images, the surface of the support 1101 may be made uneven to such an extent that charging carriers do not substantially decrease. Such unevenness provided on the surface of the support 1101 can be formed by any known methods disclosed in Japanese Patent Application Laid-Open No. 60-168156, No. 60-178457, No. 60-225854 and so forth.
As another method for more effectively canceling the faulty images due to interference fringes occurring when the coherent light such as laser light is used, the surface of the support 1101 may be made uneven by making a plurality of sphere-traced concavities on the surface of the support 1101 to such an extent that charging carriers do not substantially decrease. The surface of the support 1101 is made more finely uneven than the resolving power required for the image-forming apparatus photosensitive member 1100 and moreover such unevenness is formed by a plurality of sphere-traced concavities.
The unevenness formed by such a plurality of sphere-traced concavities provided on the surface of the support 1101 can be produced by a known method disclosed in Japanese Patent Application Laid-Open No. 61-231561.
As still another method for more effectively canceling the faulty images due to interference fringes occurring when the coherent light such as laser light is used, an interference preventive layer or region such as a light absorption layer may be provided in the photosensitive layer 1102 or beneath the photosensitive layer 1102.
Photoconductive Layer
In the image-forming apparatus photosensitive member, in order to effectively achieve its object, the photoconductive layer 1103 formed on the support 1101, or optionally on a subbing layer (not shown), and constituting part of the photosensitive layer 1102 is formed by a vacuum-deposition deposited-film formation process under conditions appropriately numerically set in accordance with film-forming parameters so that the desired characteristics can be obtained.
Stated specifically, it may be formed by, e.g., a thin-film deposition process such as glow discharging (including AC discharge CVD such as low-frequency CVD, high-frequency CVD or microwave CVD, or DC discharge CVD), sputtering, vacuum metallizing (vacuum deposition), ion plating, photo-assisted CVD or thermal CVD.
Any of these thin-film deposition processes may appropriately be selected according to factors such as the conditions for manufacture, the extent of a load on capital investment in equipment, the scale of manufacture and the properties and performances desired on image-forming apparatus photosensitive members produced. Glow discharging is preferred in view of its relative easiness to control conditions in the manufacture of image-forming apparatus photosensitive members having the desired performances.
When the photoconductive layer 1103 is formed by glow discharging, basically an Si-feeding material gas capable of feeding silicon atoms (Si), and an H-feeding material gas capable of feeding hydrogen atoms (H) and/or an X-feeding material gas capable of feeding halogen atoms (X) may be introduced in the desired gaseous state into a reactor whose inside can be evacuated, and glow discharge may be caused to take place in the reactor so that the layer comprised of a-Si:H,X is formed on a prescribed support previously set at a prescribed position.
In the image-forming apparatus photosensitive member, the photoconductive layer 1103 is required to contain hydrogen atoms and/or halogen atoms. This is important in order to compensate unbonded arms of silicon atoms in the layer and to improve layer quality, in particular, to improve photoconductivity and charge retentivity. Accordingly, the content of hydrogen atoms or halogen atoms or the total content of hydrogen atoms and halogen atoms may preferably be in a content of from 10 to 30 atom %, and more preferably form 15 to 25 atom % based on the total of silicon atoms and hydrogen atoms and/or halogen atoms.
The material that can serve as the Si-feeding gas used in the image-forming apparatus photosensitive member may include gaseous or gasifiable silicon hydrides (silanes) as those effectively usable. In view of readiness in handling for layer formation and Si-feeding efficiency, the material may preferably include SiH4 and Si2H6.
To structurally incorporate the hydrogen atoms into the photoconductive layer 1103 to be formed, and in order to make it more easy to control the percentage of the hydrogen atoms to be incorporated, and further to obtain preferable film properties, the films may preferably be formed using the above gases with which H2 and/or He or a gas of a silicon compound containing hydrogen atoms is further mixed in a desired quantity.
These gases may be used not only alone, but also in the form of a mixture of some kinds in a prescribed mixing proportion.
A material effective as a material gas for feeding halogen atoms used in the image-forming apparatus photosensitive member may preferably include gaseous or gasifiable halogen compounds as exemplified by halogen gases, halides, halogen-containing interhalogen compounds and silane derivatives substituted with a halogen.
The material may also include gaseous or gasifiable. halogen-containing silicon hydride compounds constituted of silicon atoms and halogen atoms, as those effectively usable.
In order to control the quantity of the hydrogen atoms and/or halogen atoms incorporated in the photoconductive layer 1103, for example, the temperature of the support 1101, the quantity of materials used to incorporate the hydrogen atoms and/or halogen atoms, the discharge power and so forth may be controlled.
In the image-forming apparatus photosensitive member, the photoconductive layer 1103 may preferably be incorporated with atoms capable of controlling its conductivity as occasion calls. The atoms capable of controlling the conductivity may be contained in the state they are evenly distributed in the photoconductive layer 1103, or partly non-uniformly distributed in the layer thickness direction.
The atoms capable of controlling the conductivity may include what are called impurities, used in the field of semiconductors. As well known, usable are atoms belonging to Group IIIb of the periodic table (Group IIIb atoms) capable of imparting p-type conductivity, or atoms belonging to Group Vb of the periodic table (Group Vb atoms) capable of imparting n-type conductivity.
These starting materials for incorporating the atoms capable of controlling the conductivity may be optionally diluted with H2 and/or He when used.
It is also effective to incorporate carbon atoms and/or oxygen atoms and/or nitrogen atoms in the photoconductive layer 1103. The carbon atoms and/or oxygen atoms and/or nitrogen atoms may evenly be distributed in the photoconductive layer, or may partly non-uniformly be distributed so as to change in its content in the layer thickness direction of the photoconductive layer.
In the image-forming apparatus photosensitive member, the thickness of the photoconductive layer 1103 may appropriately be determined as desired from the viewpoints of the desired electrophotographic performances to be obtained and economical advantages. The layer may preferably be formed in a thickness of from 20 to 50 xcexcm. more preferably from 23 to 45 xcexcm, and most preferably from 25 to 40 xcexcm.
In order to form the desired photoconductive layer 1103 that can achieve what is aimed in the image-forming apparatus photosensitive member and has the desired film properties, the mixing proportion of Si-feeding gas and dilute gas, the gas pressure inside the reactor, the discharge power and the support temperature may be appropriately set.
The above conditions can not independently separately be determined. Optimum values may preferably be determined on the basis of mutual and systematic relationship so that the photosensitive member having the desired properties can be formed.
Surface Layer
In the image-forming apparatus photosensitive member, the surface layer 1104 may preferably be further formed on the photoconductive layer 1103 formed on the support 1101 in the manner as described above. This surface layer 1104 has a free surface 1106, and is provided so that what is aimed in the image-forming apparatus photosensitive member can be achieved chiefly with regard to moisture resistance, performance on continuous repeated use, electrical breakdown strength, service environmental properties and running performance.
The surface layer 1104 may preferably be formed using an amorphous silicon (a-Si) type material, or any of materials such as an amorphous silicon containing a hydrogen atom (H) and/or a halogen atom (X) and further containing a carbon atom (hereinafter xe2x80x9ca-SiC:H,Xxe2x80x9d), an amorphous silicon containing a hydrogen atom (H) and/or a halogen atom (X) and further containing an oxygen atom (hereinafter xe2x80x9ca-SiO:H,Xxe2x80x9d), an amorphous silicon containing a hydrogen atom (H) and/or a halogen atom (X) and further containing a nitrogen atom (hereinafter xe2x80x9ca-SiN:H,Xxe2x80x9d), and an amorphous silicon containing a hydrogen atom (H) and/or a halogen atom (X) and further containing at least one of a carbon atom, an oxygen atom and a nitrogen atom (hereinafter xe2x80x9ca-SiCON:H,Xxe2x80x9d).
In the image-forming apparatus photosensitive member, in order to effectively achieve the object thereof, the surface layer 1104 is formed by a vacuum-deposition deposited film forming process under conditions appropriately numerically set in accordance with film forming parameters so as to achieve the desired performances. Stated specifically, it may be formed by any thin-film deposition process such as glow discharging (including AC discharge CVD such as low-frequency CVD, high-frequency CVD or microwave CVD, and DC discharge CVD), sputtering, vacuum metallizing, ion plating. photo-assisted CVD and thermal CVD.
These thin-film deposition processes are employed under appropriate selection according to the conditions for manufacture, the extent of a load on capital investment in equipment, the scale of manufacture and the properties and performances desired on image-forming apparatus photosensitive members produced. In view of productivity of photosensitive members, it is preferable to use the same deposition process as that for the photoconductive layer.
When, for example, the surface layer 1104 comprised of a-SiC;H,X is formed by glow discharging, basically an Si-feeding material gas capable of feeding silicon atoms (Si), a C-feeding material gas capable of feeding carbon atoms (C), and an H-feeding material gas capable of feeding hydrogen atoms (H) and/or an X-feeding material gas capable of feeding halogen atoms (X) may be introduced in the desired gaseous state into a reactor whose inside can be evacuated, and glow discharge may be caused to take place in the reactor so that the layer comprised of a-SiC:H,X is formed on the support 1101 previously set at a given position and on which the photoconductive layer 1103 has been formed.
When the surface layer is formed of a-SiC as a main constituent, its carbon content may preferably be in the range of from 30% to 90% based on the total of silicon atoms and carbon atoms.
In the image-forming apparatus photosensitive member, the surface layer 1104 is required to contain hydrogen atoms and/or halogen atoms. This is important in order to compensate unbonded arms of the silicon atoms and to improve layer quality, in particular, to improve photoconductivity and charge retentivity. The hydrogen atoms may usually be in a content of from 30 to 70 atom %, preferably from 35 to 65 atom %, and more preferably from 40 to 60 atom %, based on the total amount of constituent atoms. The fluorine atoms may usually be in a content of from 0.01 to 15 atom %, preferably from 0.1 to 10 atom %, and most preferably from 0.6 to 4 atom %.
Any defects or imperfections (comprised chiefly of dangling bonds of silicon atoms or carbon atoms) present inside the surface layer are known to have ill influences on the properties required for image-forming apparatus photosensitive members. For example, charging performance may deteriorate because of the injection of charges from the free surface into the photoconductive layer; charging performance may vary because of changes in surface structure in a service environment, e.g., in an environment of high humidity; and the injection of charges into the surface layer from the photoconductive layer at the time of corona charging or irradiation by light may cause a phenomenon of afterimages during repeated use because of entrapment of charges in the defects inside the surface layer.
The controlling of the hydrogen content in the surface layer so as to be 30 atom % or more brings about a great decrease in the defects inside the surface layer, so that improvements can be achieved in respect of electrical properties and high-speed continuous-service performance. On the other hand, if the hydrogen content in the surface layer is more than 70 atom %, the hardness of the surface layer tends to lower, resulting in a lowering of running performance.
The controlling of fluorine atom content in the surface layer so as to be within the range of 0.01 atom % or more also enables more effective achievement of the generation of the bonds between silicon atoms and carbon atoms in the surface layer.
As a function of the fluorine atoms in the surface layer, it is also possible to effectively prevent the bonds between silicon atoms and carbon atoms from breaking because of damage caused by coronas or the like. On the other hand, if the fluorine atom content in the surface layer is more than 15 atom %, it becomes almost ineffective to generate the bonds between silicon atoms and carbon atoms in the surface layer and to prevent the bonds between silicon atoms and carbon atoms from breaking.
Moreover, residual potential and image memory may become remarkably seen because the excessive fluorine atoms inhibit the mobility of carriers in the surface layer.
The fluorine content and hydrogen content in the surface layer may be controlled according to the flow rate of H2 gas, the support temperature, the discharge power and the gas pressure.
The surface layer 1104 in the image-forming apparatus photosensitive member may usually be formed in a thickness of from 0.01 to 3 xcexcm, preferably from 0.05 to 2 xcexcm, and more preferably from 0.1 to 1 xcexcm. If the layer thickness is smaller than 0.01 xcexcm, the surface layer may become lost because of friction or the like during the use of the photosensitive member. If it is larger than 3 xcexcm, a lowering of electrophotographic performance such as an increase in residual potential may occur.
The surface layer 1104 in the image-forming apparatus photosensitive member is carefully formed so that the required performances can be obtained as desired. From the structural viewpoint, the material constitute of any element of Si, C and/or N and/or O and R and/or X takes the form of from crystalline to amorphous depending on the conditions for its formation. From the viewpoint of electric properties, it exhibits the property of from conductive to semiconductive and up to insulating, and also the property of from photoconductive to nonphotoconductive. Accordingly, in the image-forming apparatus photosensitive member, the conditions for its formation are severally selected as desired so that a compound having the desired properties as intended can be formed.
For example, in order to provide the surface layer 1104 mainly for the purpose of improving its breakdown strength, the compound is prepared as a non-single-crystal material having a remarkable electrical insulating behavior in the service environment. When the surface layer is provided mainly for the purpose of improving the performance on continuous repeated use and service environmental properties, the compound is formed as a non-single-crystal material having become milder in its degree of the above electrical insulating properties to a certain extent and having a certain sensitivity to the light with which the layer is irradiated.
When the surface layer 1104 is formed, it is also preferable to control its resistance value appropriately on the one hand in order to prevent smeared images from being caused by a low resistance of the surface layer or prevent the layer from being affected by residual potential, and on the other hand in order to improve charging efficiency.
In the image-forming apparatus photosensitive member, a blocking layer (a lower surface layer) having a smaller content of carbon atoms, oxygen atoms and nitrogen atoms than the surface layer may further be provided between the photoconductive layer and the surface layer. This is effective for improving performances such as charging performance.
Between the surface layer 1104 and the photoconductive layer 1103, there may also be provided with a region in which the content of carbon atoms and/or oxygen atoms and/or nitrogen atoms changes in the manner that it decreases toward the photoconductive layer 1103. This makes it possible to improve the adherence between the surface layer and the photoconductive layer and to lessen an influence of interference due to reflected light at the interface between the layers.
Charge Injection Block Layer
In the image-forming apparatus photosensitive member, it is more effective to provide between the conductive support and the photoconductive layer a charge injection block layer having the function to block the injection of charges from the conductive support side.
The charge injection block layer has polarity dependence that it has the function to prevent charges from being injected from the support side to the photoconductive layer side when the photosensitive layer is subjected to charging in a certain polarity on its free surface, and exhibits no such function when subjected to charging in a reverse polarity. In order to impart such function, atoms capable of controlling its conductivity are incorporated in a relatively large content compared with those in the photoconductive layer.
The atoms capable of controlling the conductivity, contained in that layer, may evenly uniformly be distributed in the layer, or may evenly be contained in the layer thickness but contained partly in such a state that they are distributed non-uniformly. In the case when they are distributed in non-uniform concentration, they may preferably be contained so as to be distributed in a larger quantity on the support side.
In any case, however, in the in-plane direction parallel to the surface of the support, it is preferable for such atoms to be evenly contained in a uniform distribution so that the properties in the in-plane direction can also be made uniform.
The atoms capable of controlling the conductivity, incorporated in the charge injection block layer, may include what is called impurities used in the field of semiconductors, and it is possible to use the periodic table Group III atoms capable of imparting p-type conductivity, or the periodic table Group V atoms capable of imparting n-type conductivity.
The atoms capable of controlling the conductivity, incorporated in the charge injection block layer in the image-forming apparatus photosensitive member, may be in an amount determined appropriately as desired so that its object can effectively be achieved.
The charge injection block layer may further be incorporated with at least one kind of carbon atoms, nitrogen atoms and oxygen atoms. This enables achievement of more improvement of the adherence between the charge injection block layer and other layers provided in direct contact with the charge injection block layer.
The carbon atoms, nitrogen atoms or oxygen atoms contained in that layer may evenly uniformly be distributed in the layer, or may evenly be contained in the layer thickness direction but contained partly in such a state that they are distributed non-uniformly.
In any case, however, in the in-plane direction parallel to the surface of the support, it is necessary for such atoms to be evenly contained in a uniform distribution so that the properties in the in-plane direction can also be made uniform.
The carbon atoms and/or nitrogen atoms and/or oxygen atoms incorporated in the whole layer region of the charge injection block layer in the image-forming apparatus photosensitive member may be in an amount determined appropriately as desired so that its object can effectively be achieved.
Hydrogen atoms and/or halogen atoms may also be contained in the charge injection block layer in the image-forming apparatus photosensitive member, which are effective for compensating unbonded arms of constituent atoms to improve film quality,
In the image-forming apparatus photosensitive member, the charge injection block layer may preferably be formed in a thickness of from 0.1 to 5 xcexcm, more preferably from 0.3 to 4 xcexcm, and most preferably from 0.5 to 3 xcexcm, in view of the achievement of the desired electrophotographic performance and also in view of economical effects.
To form the charge injection block layer in the image-forming apparatus photosensitive member, the same vacuum deposition process as in the formation of the photoconductive layer described previously may be employed.
In addition to the foregoing, in the image-forming apparatus photosensitive member, the photosensitive layer 1102 may preferably have, on its side of the support 1101, a layer region in which at least aluminum atoms, silicon atoms and hydrogen atoms and/or halogen atoms are contained in such a state that they are distributed non-uniformly in the layer thickness direction.
In the image-forming apparatus photosensitive member, for the purpose of more improving the adherence between the support 1101 and the photoconductive layer 1103 or charge injection block layer 1105, an adherent layer may be provided which is formed of, e.g., Si3N4, SO2, SiO, or an amorphous material mainly composed of silicon atoms and containing hydrogen atoms and/or halogen atoms and carbon atoms and/or oxygen atoms and/or nitrogen atoms. A light absorption layer may also be provided for preventing occurrence of interference fringes due to the light reflecting from the support.
Production Apparatus
The photosensitive member as described above, used both in the present invention and in conventional methods and apparatus may be produced by using a known CVD apparatus as described below. FIG. 4 illustrates the construction of an example of an apparatus used to produce the photosensitive member of the present invention and the conventional photosensitive member by high-frequency plasma-assisted CVD making use of RF bands (hereinafter xe2x80x9cRF-PCVDxe2x80x9d).
This apparatus is constituted chiefly of a deposition system 2100, a material gas feed system 2220 and an exhaust system (not shown) for evacuating the inside of a reactor 2111.
In the reactor 2111 in the deposition system 2100, a cylindrical support 2112, a support heater 2113 and a material gas feed pipe 2114 are provided. A high-frequency matching box 2115 is also connected to the reactor.
The material gas feed system 2220 is constituted of gas cylinders 2221 to 2226 for material gases such as SiH4, H2, CH4, B2H6 and PH3, valves 2231 to 2236, 2241 to 2246 and 2251 to 2256, and mass flow controllers 2211 to 2216. The gas cylinders for the respective material gases are connected to the gas feed pipe 2114 in the reactor 2111 through a valve 2260.
FIG. 5 also illustrates the construction of an example of an apparatus used to produce the photosensitive member of the present invention and the conventional photosensitive member by high-frequency plasma-assisted CVD making use of VHF bands (hereinafter xe2x80x9cVHF-PCVDxe2x80x9d).
The deposition system 2100 in the apparatus shown in FIG. 4 is replaced with a deposition system 3100 as shown in FIG. 5, to connect it to the material gas feed system 2200. Thus, a production apparatus used in VHF-PCVD is set up.
This production apparatus is constituted chiefly of an inside-evacuatable reactor 3111 having a vacuum-sealed structure, a material gas feed system 2200 and an exhaust system (not shown) for evacuating the inside of the reactor.
In the reactor 3111, cylindrical supports 3112, support heaters 3113, a material gas feed pipe 3114 and an electrode are provided. A high-frequency matching box 3120 is also connected to the electrode.
The inside of the reactor 3111 is connected to a diffusion pump (not shown) through an exhaust tube 3121. In the reactor, space 3130 surrounded by the cylindrical supports 3112 forms a discharge space.
In recent years, with expansion of networks in offices and with spread of information made rich in variety, color image formation is becoming popular in printers and copying machines, too. In particular, with the expansion of the amount of information, color printers and color copying machines are sought to be made having high-speed.
Conventionally, in photosensitive members which are latent-image-bearing members of such color copying machines, OPC photosensitive members have widely been used as stated previously. The OPC photosensitive members, however, have a low hardness and may be abraded, so that the photosensitive members have had to be more often replaced as machines have higher speed. Accordingly, in respect of studies on high-speed copying machines making use of OPC photosensitive members, it has been studied to make them have higher hardness to prevent their abrasion so as to cope with high-speed copying.
On the other hand, in image-forming apparatus making use of the a-Si photosensitive members, they have a high hardness and hence can solve the problem of replacement of photosensitive members because of drum abrasion occurring in the OPC photosensitive members. Also, there is an advantage that they have a good dot reproducibility and can provide copies having a high image quality. There are, however, some problems in the employment of the a-Si in digital color copying machines.
The a-Si photosensitive members may cause the formation of smeared images in conditions of high temperature and high humidity and further may cause another problem of a change in surface potential because of variations of temperature. To solve these problems, a drum heater is put inside the photosensitive member to control temperature to a constant level.
Meanwhile, toners for color copying machines are so made up that a plurality of color toners are multiply fixed, and hence the softening point of the toners has been set low.
Where toners having a high softening point are used, their color mixing performance in fixing assemblies may lower to cause a problem in color reproducibility. Where such toners having a high softening point are used in high-speed full-color copying machines, a great mechanical shear may act at the part where rollers at cleaner and transfer zones come into contact with the photosensitive member and also the photosensitive member may generate heat so greatly as to make the toners tend to melt on the photosensitive member.
This may occur more remarkably especially when the photosensitive member is temperature-controlled by the drum heater, and may cause a problem of melt adhesion that the toners adhere to the photosensitive member and a problem of filming that toner resin accumulates uniformly on the photosensitive member surface.
Thus, also in the case when a-Si photosensitive members are used, it has been necessary for the photosensitive members to be put to maintenance, making it impossible to well bring out the advantage of long lifetime the a-Si photosensitive members have originally.
Accordingly, in the case when the a-Si photosensitive member are set in color copying machines, it has been considered necessary to newly provide toners that can achieve both the prevention of melt adhesion and filming to the photosensitive member and the color reproducibility in fixing.
In addition, where photosensitive members axe set in tandem-type full-color copying machines, the photosensitive members are restricted to a certain size because of the internal space of the apparatus. As the result, respective assemblies having the functions of charging, exposure, development, transfer, cleaning and charge elimination are restricted to certain sizes.
Especially when the width of a charging assembly is restricted, no sufficient surface potential may be obtained on the photosensitive member, so that no high-density images may be obtained. Accordingly, it is required to provide a toner and a developing method which can obtain a sufficient image density even in low-potential development. Also, even in a system having achieved such low-potential development, it is required to establish a toner and a developing method which can provide images having good color reproducibility and high image quality.
Stated additionally, another advantage in using the a-Si photosensitive members in full-color copying machines is that images can be formed in a high image quality. The a-Si photosensitive members can well restrict the level of dots produced by imagewise exposure, and can form images in a high image quality.
In an attempt to merely make color toners have a small particle diameter, the toners may have a large charge quantity and such toners commonly tend to participate in development in a small quantity. This may act disadvantageously on the low-potential development on a-Si photosensitive members. Hence, it is urgently sought to newly provide an image-forming method and an image-forming apparatus which can form images in a high minuteness and a high image quality, using toners showing a high developing performance.
The present invention was made taking account of the above circumstances. Accordingly, an object of the present invention is to provide an image-forming method and an image-forming apparatus which can materialize high-minuteness, high-image-quality and high-speed image formation while preventing deterioration of photosensitive members and improving their running performance.
To achieve the above object, the present invention provides an image-forming method used in an image-forming apparatus having;
four image-forming units making use of a first toner, a second toner, a third toner and a fourth toner which have colors different from one another, for forming toner images on a transfer medium; and
a heat-and-pressure fixing means for performing heat-and-pressure treatment on the transfer medium having the toner images thereon;
the four image-forming units each having:
a photosensitive member having an amorphous silicon or non-single-crystal silicon layer;
a charging means for charging the photosensitive member electrostatically;
an exposure means for exposing the photosensitive member to form an electrostatic latent image thereon; and
a developing means having a developing sleeve for developing the electrostatic latent image formed on the photosensitive member;
the photosensitive member having a diameter of from 20 mm to 80 mm;
after charging the photosensitive member with the charging means, the electrostatic latent image being formed by exposure with the exposure means, and, at a development position in unexposed areas, the photosensitive member being made to have a surface potential of from 300 V to 450 V as an absolute value;
the developing means having a two-component developer containing the toner and a carrier:
the photosensitive member and the developing sleeve being so disposed as to have a minimum gap between them of from 350 xcexcm to 800 xcexcm;
while the developing sleeve rotates at a peripheral speed from 1.1 times to 4.0 times the peripheral speed of the photosensitive member, the electrostatic latent image being developed with a magnetic brush of the two-component developer to form a toner image on the photosensitive member:
the first toner, second toner, third toner and fourth toner being selected from the group consisting of a non-magnetic yellow toner, a non-magnetic magenta toner, a non-magnetic cyan toner and a non-magnetic black toner;
the non-magnetic yellow toner, non-magnetic magenta toner, non-magnetic cyan toner and non-magnetic black toner having negative chargeability and each having a weight-average particle diameter of from 4.0 xcexcm to 10.0 xcexcm;
the carrier of the two-component developer having a 50% average particle diameter of from 10 xcexcm to 80 xcexcm; and
where a coloring power of the toner of each color is defined as image density D0.5 measured after being fixed once when a quantity of unfixed toner on a transfer medium, M/S, is 0.5 mg/cm2 and the coloring power of the non-magnetic yellow toner is represented by D0.5Y, the coloring power of the non-magnetic magenta toner by D0.5M, the coloring power of the non-magnetic cyan toner by D0.5C and the coloring power of the non-magnetic black toner by D0.5Bk, each of D0.5Y, D0.5M, D0.5C and D0.5Bk being each from 1.0 to 1.8 as image density, and, where the coloring power of the toner showing the maximum coloring power among the three colors of yellow, magenta and cyan is represented by D0.5max, and the coloring power of the toner showing the minimum coloring power by D0.5min. a difference between D0.5max and D0.5min being 0.5 or less.
The present invention also provides an image-forming method for forming a full-color image or a multi-color image on a transfer medium by:
transferring to the transfer medium a first toner image formed in a first image-forming unit;
transferring to the transfer medium having the first toner image a second toner image formed in a second image-forming unit;
transferring to the transfer medium having the first and second toner images a third toner image formed in a third image-forming unit;
transferring to the transfer medium having the first, second and third toner images a fourth toner image formed in a fourth image-forming unit; and
transporting to a heat-and-pressure fixing means the transfer medium having the first, second, third and fourth toner images to effect heat-and-pressure fixing;
(A) the formation of the first toner image in the first image-forming unit:
(i) comprising at least a first charging step of electrostatically charging a first photosensitive member having an amorphous silicon or non-single-crystal silicon layer, a first exposure step, and a first developing step having a first developing sleeve;
(ii) the first photosensitive member having a diameter of from 20 mm to 80 mm; the first photosensitive member being charged in the first charging step from 300 V to 450 V as an absolute value at its developing zone opposite to the first developing sleeve; and thereafter a first electrostatic latent image being formed on the first photosensitive member by exposure made in the first exposure step;
(iii) in the first developing step, a magnetic brush of a two-component developer containing a first toner and a first magnetic carrier being formed on the first developing sleeve;
(iv) the first photosensitive member and the first developing sleeve being so disposed as to have a minimum gap between them of from 350 xcexcm to 800 xcexcm;
(v) the first electrostatic latent image being developed with the magnetic brush of the two-component developer while the first developing sleeve is rotated at a peripheral speed from 1.1 times to 4.0 times the peripheral speed of the first photosensitive member, to form the first toner image on the first photosensitive member
(B) the formation of the second toner image in the second image-forming unit:
(i) comprising at least a second charging step of electrostatically charging a second photosensitive member having an amorphous silicon or non-single-crystal silicon layer, a second exposure step, and a second developing step having a second developing sleeve;
(ii) the second photosensitive member having a diameter of from 20 mm to 80 mm; the second photosensitive member being charged in the second charging step from 300 V to 450 V as an absolute value at its developing zone opposite to the second developing sleeve; and thereafter a second electrostatic latent image being formed on the second photosensitive member by exposure made in the second exposure step;
(iii) in the second developing step, a magnetic brush of a two-component developer containing a second toner and a second magnetic carrier being formed on the second developing sleeve;
(iv) the second photosensitive member and the second developing sleeve being so disposed as to have a minimum gap between them of from 350 xcexcm to 800 xcexcm;
(v) the second electrostatic latent image being developed with the magnetic brush of the two-component developer while the second developing sleeve is rotated at a peripheral speed from 1.1 times to 4.0 times the peripheral speed of the second photosensitive member, to form the second toner image on the second photosensitive member;
(C) the formation of the third toner image in the third image-forming unit:
(i) comprising at least a third charging step of electrostatically charging a third photosensitive member having an amorphous silicon or non-single-crystal silicon layer, a third exposure step, and a third developing step having a third developing sleeve;
(ii) the third photosensitive member having a diameter of from 20 mm to 80 mm; the third photosensitive member being charged in the third charging step from 300 V to 450 V as an absolute value at its developing zone opposite to the third developing sleeve; and thereafter a third electrostatic latent image being formed on the third photosensitive member by exposure made in the third exposure step;
(iii) in the third developing step, a magnetic brush of a two-component developer containing a third toner and a third magnetic carrier being formed on the third developing sleeve;
(iv) the third photosensitive member and the third developing sleeve being so disposed as to have a minimum gap between them of from 350 xcexcm to 800 xcexcm;
(v) the third electrostatic latent image being developed with the magnetic brush of the two-component developer while the third developing sleeve is rotated at a peripheral speed from 1.1 times to 4.0 times the peripheral speed of the third photosensitive member, to form the third toner image on the third photosensitive member;
(D) the formation of the fourth toner image in the fourth image-forming unit:
(i) comprising at least a fourth charging step of electrostatically charging a fourth photosensitive member having an amorphous silicon or non-single-crystal silicon layer, a fourth exposure step, and a fourth developing step having a fourth developing sleeve;
(ii) the fourth photosensitive member having a diameter of from 20 mm to 80 mm; the fourth photosensitive member being charged in the fourth charging step from 300 V to 450 V as an absolute value at its developing zone opposite to the fourth developing sleeve; and thereafter a fourth electrostatic latent image being formed on the fourth photosensitive member by exposure made in the fourth exposure step;
(iii) in the fourth developing step, a magnetic brush of a two-component developer containing a fourth toner and a fourth magnetic carrier being formed on the fourth developing sleeve;
(iv) the fourth photosensitive member and the fourth developing sleeve being so disposed as to have a minimum gap between them of from 350 xcexcm to 800 xcexcm;
(v) the fourth electrostatic latent image being developed with the magnetic brush of the two-component developer while the fourth developing sleeve is rotated at a peripheral speed from 1.1 times to 4.0 times the peripheral speed of the fourth photosensitive member, to form the fourth toner image on the fourth photosensitive member, and
(E) the first toner, second toner, third toner and fourth toner having color tones different from one another, and each being selected from the group consisting of a non-magnetic yellow toner, a non-magnetic magenta toner, a non-magnetic cyan toner and a non-magnetic black toner;
(a) the non-magnetic yellow toner, non-magnetic magenta toner, non-magnetic cyan toner and non-magnetic black toner having positive chargeability and each having a weight-average particle diameter of from 4.0 xcexcM to 10.0 xcexcm;
(b) the magnetic carrier of the two-component developer having a 50% volume-average particle diameter of from 10 xcexcm to 80 xcexcm; and
(c) where a coloring power of the toner of each color is defined as image density DO.5 measured after being fixed once when a quantity of unfixed toner on a transfer medium, M/S, is 0.5 mg/cm2 and the coloring power of the non-magnetic yellow toner is represented by D0.5Y, the coloring power of the non-magnetic magenta toner by D0.5M, the coloring power of the non-magnetic cyan toner by D0.5C and the coloring power of the non-magnetic black toner by D0.5Bk, each of D0.5Y, D0.5M, D0.5C and D0.5Bk being from 1.0 to 1.8 as image density, and, where the coloring power of the toner showing the maximum coloring power among the three colors of yellow, magenta and cyan is represented by D0.5max, and the coloring power of the toner showing the minimum coloring power by D0.5min, a difference between D0.5max and D0.5min being from 0 to 0.5.
The present invention still also provides an image-forming method for forming a full-color image on a transfer medium by:
transferring to the transfer medium a first toner image formed in a first image-forming unit;
transferring to the transfer medium having the first toner image a second toner image formed in a second image-forming unit;
transferring to the transfer medium having the first and second toner images a third toner image formed in a third image-forming unit:
transferring to the transfer medium having the first, second and third toner images a fourth toner image formed in a fourth image-forming unit; and
fixing the first, second, third and fourth toner images to the transfer medium by heat-and-pressure fixing;
(A) the first image-forming unit:
(i) comprising at least a first photosensitive drum, a first charging means for charging the first photosensitive drum electrostatically, a first exposure means for forming on the photosensitive drum thus charged a first electrostatic latent image by exposure, and a first developing means for developing the electrostatic latent image at a developing zone;
(ii) the first photosensitive drum having an amorphous silicon layer as a photosensitive layer, having a diameter of from 20 mm to 80 mm, and having at unexposed areas in the developing zone a surface potential of from 300 V to 450 V as an absolute value; and
(iii) the first developing means having a one-component developer containing a first toner and a first developing sleeve for transporting the developer to the developing zone;
the first photosensitive drum and the first developing sleeve being so disposed as to either contact each other or maintain a minimum gap between them;
the first electrostatic latent image being developed with the one-component developer while in a case of contact development the first developing sleeve is rotated at a peripheral speed from 1.05 times to 2.0 times the peripheral speed of the first photosensitive drum and in a case of non-contact development the first developing sleeve is rotated at a peripheral speed from 1.1 times to 4.0 times the peripheral speed of the first photosensitive drum, to form the first toner image on the first photosensitive drum;
(B) the second image-forming unit:
(i) comprising at least a second photosensitive drum, a second charging means for charging the second photosensitive drum electrostatically, a second exposure means for forming on the photosensitive drum thus charged a second electrostatic latent image by exposure, and a second developing means for developing the electrostatic latent image at a developing zone;
(ii) the second photosensitive drum having an amorphous silicon layer as a photosensitive layer, having a diameter of from 20 mm to 80 mm, and having at unexposed areas in the developing zone a surface potential of from 300 V to 450 V as an absolute value; and
(iii) the second developing means having a one-component developer containing a second toner and a second developing sleeve for transporting the developer to the developing zone;
the second photosensitive drum and the second developing sleeve being so disposed as to either contact each other or maintain a minimum gap between them;
the second electrostatic latent image being developed with the one-component developer while in the case of contact development the second developing sleeve is rotated at a peripheral speed from 1.05 times to 2.0 times the peripheral speed of the second photosensitive drum and in the case of non-contact development the second developing sleeve is rotated at a peripheral speed from 1.1 times to 4.0 times the peripheral speed of the second photosensitive drum, to form the second toner image on the second photosensitive drum;
(C) the third image-forming unit:
(i) comprising at least a third photosensitive drum, a third charging means for charging the third photosensitive drum electrostatically, a third exposure means for forming on the photosensitive drum thus charged a third electrostatic latent image by exposure, and a third developing means for developing the electrostatic latent image at a developing zone;
(ii) the third photosensitive drum having an amorphous silicon layer as a photosensitive layer, having a diameter of from 20 mm to 80 mm, and having at unexposed areas in the developing zone a surface potential of from 300 V to 450 V as an absolute value; and
(iii) the third developing means having a one-component developer containing a third toner and a third developing sleeve for transporting the developer to the developing zone;
the third photosensitive drum and the third developing sleeve being so disposed as to either contact each other or maintain a minimum gap between them;
the third electrostatic latent image being developed with the one-component developer while in a case of contact development the third developing sleeve is rotated at a peripheral speed from 1.05 times to 2.0 times the peripheral speed of the third photosensitive drum and in a case of non-contact development the third developing sleeve is rotated at a peripheral speed from 1.1 times to 4.0 times the peripheral speed of the third photosensitive drum, to form the third toner image on the third photosensitive drum;
(D) the fourth image-forming unit:
(i) comprising at least a fourth photosensitive drum, a fourth charging means for charging the fourth photosensitive drum electrostatically, a fourth exposure means for forming on the photosensitive drum thus charged a fourth electrostatic latent image by exposure, and a fourth developing means for developing the electrostatic latent image at a developing zone;
(ii) the fourth photosensitive drum having an amorphous silicon layer as a photosensitive layer, having a diameter of from 20 mm to 80 mm, and having at unexposed areas in the developing zone a surface potential of from 300 V to 450 V as an absolute value; and
(iii) the fourth developing means having a one-component developer containing a fourth toner and a fourth developing sleeve for transporting the developer to the developing zone;
the fourth photosensitive drum and the fourth developing sleeve being so disposed as to either contact each other or maintain a minimum gap between them;
the fourth electrostatic latent image being developed with the one-component developer while in a case of contact development the fourth developing sleeve is rotated at a peripheral speed from 1.05 times to 2.0 times the peripheral speed of the fourth photosensitive drum and in a case of non-contact development the fourth developing sleeve is rotated at a peripheral speed from 1.1 times to 4.0 times the peripheral speed of the fourth photosensitive drums to form the fourth toner image on the fourth photosensitive drum; and
(E) the first toner, second toner, third toner and fourth toner having color tones different from one another, and each being selected from the group consisting of a non-magnetic yellow toner, a non-magnetic magenta toner, a non-magnetic cyan toner and a non-magnetic black toner;
(a) the non-magnetic yellow toner, non-magnetic magenta toner, non-magnetic cyan toner and non-magnetic black toner being each a negatively chargeable toner containing a binder resin and a colorant: each having a weight-average particle diameter of from 4.0 xcexcm to 10.0 xcexcm; and
(b) each toner having a coloring power of from 1.0 to 1.8, and a difference between the coloring power of the toner showing the maximum coloring power among the three colors of yellow, magenta and cyan and the coloring power of the toner showing the minimum coloring power among them being from 0 to 0.5.
The present invention further provides an image-forming method for forming a full-color image or a multi-color image on a transfer medium by:
transferring to the transfer medium a first toner image formed in a first image-forming unit;
transferring to the transfer medium having the first toner image a second toner image formed in a second image-forming unit;
transferring to the transfer medium having the first and second toner images a third toner image formed in a third image-forming unit;
transferring to the transfer medium having the first, second and third toner images a fourth toner image formed in a fourth image-forming unit; and
transporting to a heat-and-pressure fixing means the transfer medium having the first, second, third and fourth toner images to effect heat-and-pressure fixing;
(A) the formation of the first toner image in the first image-forming unit:
(i) comprising at least a first charging step of electrostatically charging a first photosensitive member having an amorphous silicon or non-single-crystal silicon layer, a first exposure step, and a first developing step having a first developing sleeve;
(ii) the first photosensitive member having a diameter of from 20 mm to 80 mm; the first photosensitive member being charged in the first charging step from 300 V to 450 V as an absolute value at its developing zone opposite to the first developing sleeve, and thereafter a first electrostatic latent image being formed on the first photosensitive member by exposure made in the first exposure step; and
(iii) in the first developing step, a one-component developer being used which contains a first toner;
the first photosensitive member and the first developing sleeve being so disposed as to either contact each other or maintain a minimum gap between them;
the first electrostatic latent image being developed with the one-component developer while in the case of contact development the first developing sleeve is rotated at a peripheral speed from 1.05 times to 2.0 times the peripheral speed of the first photosensitive drum and in the case of non-contact development the first developing sleeve is rotated at a peripheral speed from 1.1 times to 4.0 times the peripheral speed of the first photosensitive drum, to form the first toner image on the first photosensitive drum;
(B) the formation of the second toner image in the second image-forming unit:
(i) comprising the method having at least a second charging step of electrostatically charging a second photosensitive member having an amorphous silicon or non-single-crystal silicon layer, a second exposure step, and a second developing step having a second developing sleeve;
(ii) the second photosensitive member having a diameter of from 20 mm to 80 mm; the second photosensitive member being charged in the second charging step from 300 V to 450 V as an absolute value at its developing zone opposing the second developing sleeve; and thereafter a second electrostatic latent image being formed on the second photosensitive member by exposure made in the second exposure step; and
(iii) in the second developing step, a one-component developer being used which contains a second toner;
the second photosensitive member and the second developing sleeve being so disposed as to either contact each other or maintain a minimum gap between them;
the second electrostatic latent image being developed with the one-component developer while in the case of contact development the second developing sleeve is rotated at a peripheral speed from 1.05 times to 2.0 times the peripheral speed of the second photosensitive drum and in the case of non-contact development the second developing sleeve is rotated at a peripheral speed from 1.1 times to 4.0 times the peripheral speed of the second photosensitive drum, to form the second toner image on the second photosensitive drum;
(C) the formation of the third toner image in the third image-forming unit;
(i) comprising the method having at least a third charging step of electrostatically charging a third photosensitive member having an amorphous silicon or non-single-crystal silicon layer, a third exposure step, and a third developing step having a third developing sleeve;
(ii) the third photosensitive member having a diameter of from 20 mm to 80 mm; the third photosensitive member being charged in the third charging step from 300 V to 450 V as an absolute value at its developing zone opposite to the third developing sleeve; and thereafter a third electrostatic latent image being formed on the third photosensitive member by exposure made in the third exposure step; and
(iii) in the third developing step, a one-component developer being used which contains a third toner;
the third photosensitive member and the third developing sleeve being so disposed as to either contact each other or maintain a minimum gap between them;
the third electrostatic latent image being developed with the one-component developer while in the case of contact development the third developing sleeve is rotated at a peripheral speed from 1.05 times to 2.0 times the peripheral speed of the third photosensitive drum and in the case of non-contact development the third developing sleeve is rotated at a peripheral speed from 1.1 times to 4.0 times the peripheral speed of the third photosensitive drum. to form the third toner image on the third photosensitive drum;
(D) the formation of the fourth toner image in the fourth image-forming unit:
(i) comprising the method having at least a fourth charging step of electrostatically charging a fourth photosensitive member having an amorphous silicon or non-single-crystal silicon layer, a fourth exposure step, and a fourth developing step having a fourth developing sleeve;
(ii) the fourth photosensitive member having a diameter of from 20 mm to 80 mm; the fourth photosensitive member being charged in the fourth charging step from 300 V to 450 V as an absolute value at its developing zone opposing the fourth developing sleeve; and thereafter a fourth electrostatic latent image being formed on the fourth photosensitive member by exposure made in the fourth exposure step; and
(iii) in the fourth developing step, a one-component developer being used which contains a fourth toner;
the fourth photosensitive member and the fourth developing sleeve being so disposed as to either contact each other or maintain a minimum gap between them;
the fourth electrostatic latent image being developed with the one-component developer while in the case of contact development the fourth developing sleeve is rotated at a peripheral speed from 1.05 times to 2.0 times the peripheral speed of the fourth photosensitive drum and in the case of non-contact development the fourth developing sleeve is rotated at a peripheral speed from 1.1 times to 4.0 times the peripheral speed of the fourth photosensitive drum, to form the fourth toner image on the fourth photosensitive drum; and
(E) the first toner, second toner, third toner and fourth toner having color tones different from one another, and each being selected from the group consisting of a non-magnetic yellow toner, a non-magnetic magenta toner, a non-magnetic cyan toner and a non-magnetic black toner;
(a) the non-magnetic yellow toner, non-magnetic magenta toner, non-magnetic cyan toner and non-magnetic black toner being positively chargeable and each having a weight-average particle diameter of from 4.0 xcexcm to 10.0 xcexcm; and
(b) where the coloring power of the toner of each color is defined as image density D0.5 measured after being fixed once when a quantity of unfixed toner on a transfer medium, M/S, is 0.5 mg/cm2 and the coloring power of the nonmagnetic yellow toner is represented by D0.5Y. the coloring power of the non-magnetic magenta toner by D0.5M, the coloring power of the nonmagnetic cyan toner by D0.5C and the coloring power of the non-magnetic black toner by D0.5Bk, each of D0.5Y, D0.5M, D0.5C and D0.5Bk being from 1.0 to 1.8 as image density, and, where the coloring power of the toner showing the maximum coloring power among the three colors of yellow, magenta and cyan is represented by D0.5max, and the coloring power of the toner showing the minimum coloring power by D0.5min, a difference between D0.5max and D0.5min being from 0 to 0.5.
The present invention still further provides image-forming apparatus used in the above image-forming methods.